This invention relates to geometric instruments, one particular application of which relates to dental instruments for and methods of preparing one or more teeth for application of a crown or bridge.
In the preparation of crowns or copings; it is essential that each crown or coping fit precisely to the margin of the prepared tooth surfaces. Otherwise, it is not possible to achieve a tight margin seal so that food particles, acids, etc., are allowed to accumulate underneath the crown and destroy the tooth. The tooth misial and distal surfaces as well as the buccal, labial and lingual surfaces, as the case may be, are prepared by grinding them on an incline (desirably 6.degree. to the vertical) to form a tapered prepared tooth configuration suitable for preparation of a wax pattern and subsequent reception of the crown or coping. Inasmuch as this grinding process is presently done by eye, however, the prepared tooth surfaces often tend to be of non-uniform shape, include undercut portions, and generally fail to provide, on a consistent basis, a prepared tooth configuration suitable for accurate preparation of the wax pattern, the non-uniform undercut portions tending to distort the wax pattern during removal thereof from a die. (The die is formed by a plaster model taken from a wax impression of the prepared teeth). In many practical cases, therefore, the crown or bridge prepared from the wax pattern does not fit precisely and must be fitted by acutally grinding it, or one or more teeth, until it can be forced into position, with resultant damage to or even destruction of a precise margin fit.
In the preparation and fitting of individual crowns, the process mentioned above generally is acceptable because each tooth is prepared and fitted with a crown independently so that the effects of inaccurately prepared tooth surfaces are reflected in the preparation and fitting of only one crown. This is not the case with dental bridges, especially bridges formed as one piece multiple crown castings, because the bridge must be prepared and fitted so that all its crowns fit precisely to the margins of several adjacent teeth, or to abutments separated by a ponic, simultaneously. Heretofore, the preparation and fitting of bridges have been done in much the same manner as the preparation and fitting of individual crowns--the crowns making up the bridge being formed independently and soldered together. Soldering, however, tends to distort the alignment of the individual crowns and is time consuming. Although one piece multiple castings now offer a means for eliminating soldering to obtain accurate alignment (and hence fit) of the individual crowns and increasing productivity, the independently prepared teeth almost invariably include sufficient non-uniformities that an accurate wax pattern is exceedingly difficult to obtain. Consequently, one piece multiple castings prepared from such wax patterns often necessitate as much or more alteration to make them fit than soldered bridges--the margin seals obtained with both types of bridges being unreliable in many instances.